There is a continuing need for improved devices for use with coin-operated apparatus such as vending machines, coin changers, telephones, etc., to distinguish genuine U.S. coins from slugs and foreign coins. Various mechanical and electrical devices have been utilized for such purpose; however, none of them successfully rejects all types of slugs or undesirable foreign coins under all conditions.
One type of system that has been proposed is the use of a pair of oscillators in a bridge circuit, with the coil of one oscillator being loaded with a genuine coin. A coin or slug inserted into the machine is made to pass through the field of the coil of the other oscillator, which either balances the bridge or not. If the coin is genuine, the bridge is balanced and an output signal, or a lack of an output signal, depending on the associated circuitry, opens a mechanical gate to allow the coin to pass into the coin receptacle and actuate the mechanism. One such device is illustrated in U.S. Pat. No. 3,059,749 to Zinke, issued Oct. 23, 1962.
In a co-pending U.S. patent application Ser. No. 43,713, filed May 30, 1979, now abandoned, by George E. Fafard and assigned to the same assignee as this invention, there is illustrated another method of utilizing an oscillator coil to distinguish between genuine U.S. coins and undesired coins or slugs. In the device shown in that application, the output from the oscillator circuit is applied to a window comparator. If the oscillator output falls within the voltage limits established by the window comparator for genuine coins, an output signal is produced. The voltage limits for the window detector are provided from a conventional voltage source. In that application, the reflecting of the coin is also measured by an LED photocell apparatus, principally to identify lead slugs. Although this device operates satisfactorily in a constant temperature environment, it is not suitable for use in locations where there are substantial temperature changes.